1. Field of the Invention
The present invention relates to a zoom lens. The present invention also relates to an image pickup apparatus such as a digital camera or a video camera that is equipped with a zoom lens.
2. Description of the Related Art
In recent years, digital cameras that pick up an image of an object using an image pickup element such as a CCD or CMOS sensor have replaced film cameras and become the mainstream. Furthermore, various categories of digital cameras ranging from popular-priced compact type cameras to advanced type cameras for professionals have been developed.
Users of such popular-priced compact digital cameras (what is called compact cameras and small-sized cameras with interchangeable lenses) generally wish to enjoy easy and simple shooting in various shooting situations anywhere at any time. Therefore, a further reduction in the size of their taking lens system is demanded.
The recent mainstream technique of slimming the camera body is the use of a collapsible lens barrel. The collapsible lens barrel is adapted to extend out of the camera body when in use, and to be housed in the camera body when not in use to facilitate portability.
Another known slimming technique is bending the optical path (optical axis) of the optical system by a reflecting optical member such as a mirror or a prism. Cameras with this design can be brought into usable state without start-up time, which cameras with a collapsible lens take (to project the lens). In addition, this design is advantageous in achieving waterproofing and dustproofing. Furthermore, with this design, cameras with very small depth can be achieved.
In digital cameras with this design, the position of the lens unit closest to the object side is fixed during zooming, and a reflecting optical member is provided in this lens unit so that the optical path is bent by reflection to extend in a vertical or horizontal direction of the camera body. Thus, the depth of the camera can be made as small as possible.
For example, Japanese Patent Application Laid-Open NO. 2006-343622 discloses a zoom lens that has a reflecting optical member provided in the first lens unit closest to the object side and includes an object side lens group having a negative refracting power including the first lens unit having a positive refracting power and a second lens unit having a negative refracting power, a third lens unit having a positive refracting power, and a fourth lens unit having a positive refracting power arranged in order from the object side. With this configuration, relatively good optical performance can be achieved while making the depth small.
To reduce the size of a zoom lens, it is preferred that the lens surface closest to the image side in a positive lens unit in a rear side lens group be a concave surface, as is the case with the zoom lens disclosed in Japanese Patent Application Laid-Open NO. 2006-343622. This design facilitates refracting off-axis rays away from the optical axis by the concave surface facing the image side, thereby facilitating a reduction in the effective size of the zoom lens relative to the size of the image pickup surface.
In the case of zoom lenses having straight optical axis (i.e. interchangeable lenses and collapsible type lenses), the aforementioned design leads to a reduction in the diameter and overall length of the zoom lenses. In the case of zoom lenses in which the optical axis is bent, the aforementioned design leads to slimming of the camera body.
If the lens surface closest to the image side in a positive lens unit is a concave surface, the size of this lens unit can be made small. Therefore, in many cases, this positive lens unit is designed to have a sufficiently high positive refracting power to contribute to the magnification change, as disclosed in Japanese Patent Application Laid-Open NO. 2006-343622.